Capacitors are basic building blocks for electronic circuits. Capacitors function principally to store charge or to add reactance to an ac circuit. When combined with other electronic devices, numerous useful circuits may be constructed. For example, when a capacitor is electrically connected to an inductor (an electromagnetic field storage device) a resonant circuit results. Such resonant circuits have numerous applications, such as for an antenna to pick-up radio frequency radiation.
Superconductivity refers to that state of metals and alloys in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature (“Tc”).
Until recently, attaining the Tc of known superconducting materials required the use of liquid helium and expensive cooling equipment. However, in 1986 a superconducting material having a Tc of 30K was announced. See, e.g., Bednorz and Muller, Possible High Tc Superconductivity in the Ba—La—Cu—O System, Z. Phys. B-Condensed Matter 64, 189-193 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors. Currently, superconducting materials having critical temperatures in excess of the boiling point of liquid nitrogen, 77K at atmospheric pressure, have been disclosed.
Superconducting compounds consisting of combinations of alkaline earth metals and rare earth metals such as barium and yttrium in conjunction with copper (known as “YBCO superconductors”) were found to exhibit superconductivity at temperatures above 77K. See, e.g., Wu, et al., Superconductivity at 93K in a New Mixed-Phase Y—Ba—Cu—O Compound System at Ambient Pressure, Phys. Rev. Lett. 58, No. 9, 908-910 (1987). In addition, high temperature superconducting compounds containing bismuth have been disclosed. See, e.g., Maeda, A New High-Tc Oxide Superconductor Without a Rare Earth Element, J. App. Phys. 37, No. 2, L209-210 (1988); and Chu, et al., Superconductivity up to 114K in the Bi—Al—Ca—Br—Cu—O Compound System Without Rare Earth Elements, Phys. Rev. Lett. 60, No. 10, 941-943 (1988). Furthermore, superconducting compounds containing thallium have been discovered to have critical temperatures ranging from 90K to 123K (the highest critical temperatures to date). See, e.g., G. Koren, A. Gupta, and R. J. Baseman, Appl. Phys. Lett. 54, 1920 (1989).
These high temperature superconductors have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices. More particularly, the applicant's assignee has successfully produced thin film thallium superconductors which are epitaxial to the substrate. See, e.g., Olson, et al., Preparation of Superconducting TlCaBaCu Thin Films by Chemical Deposition, Appl. Phys. Lett. 55, No. 2, 189-190 (1989), incorporated herein by reference. Techniques for fabricating and improving thin film thallium superconductors are described in the following patent and copending applications: Olson, et al., U.S. Pat. No. 5,071,830, issued Dec. 10, 1991; Controlled Thallous Oxide Evaporation for Thallium Superconductor Films and Reactor Design, Ser. No. 516,078, filed Apr. 27, 1990; In Situ Growth of Superconducting Films, Ser. No. 598,134, filed Oct. 16, 1990; Passivation Coating for Superconducting Thin Film Device, Ser. No. 697,660, filed May 8, 1991; and Fabrication Process for Low Loss Metallizations on Superconducting Thin Film Devices, Ser. No. 697,960, filed May 8, 1991, all incorporated herein by reference.
High temperature superconducting films are now routinely manufactured with surface resistances significantly below 500μΩ measured at 10 GHz and 77K. These films may be formed into resonant circuits. Such superconducting films when formed as resonators have an extremely high quality factor (“Q”). The Q of a device is a measure of its lossiness or power dissipation. In theory, a device with zero resistance (i.e. a lossless device) would have a Q of infinity. Superconducting devices manufactured and sold by applicant's assignee routinely achieve a Q in excess of 15,000. This is high in comparison to a Q of several hundred for the best known non-superconducting conductors having similar structure and operating under similar conditions.
Superconducting thin film resonators have the desirable property of having very high energy storage in a relatively small physical space. The superconducting resonators are compact and lightweight. Another benefit of superconductors is that relatively long circuits may be fabricated without introducing significant loss. For example, an inductor coil of a detector circuit made from superconducting material can include more turns than a similar coil made of non-superconducting material without experiencing a significant increase in loss as would the non-superconducting coil. Therefore, the superconducting coil has increased signal pick-up and is much more sensitive than the non-superconducting coil.
Typical resonant circuits are generally limited in their application due to their signal-to-noise ratios (“SNR”). For example, the SNR in a pickup coil of a MRI detector is a limiting factor for low-field MRI systems. Although the low-field MRI systems have a number of advantages over high-field MRI (including cost, site requirements, patient comfort and tissue contrast), they have not yet found wide-spread use in the U.S. because, in part, of their lower SNR. Resonant circuits made from superconductors improve SNR for low-field human imaging. Therefore, an appropriate superconducting resonant circuit, depending on the field level, coil type, and imaging region, will enable wide-spread use of low-field MRI.
An MRI detector including a low temperature superconducting coil and capacitor has been described. See, e.g., Rollwitz, U.S. Pat. No. 3,764,892, issued Oct. 9, 1973. In addition, resonant circuits for use as MRI detectors which include high temperature superconducting coils and non-superconducting capacitors have been described. See, e.g., Wang, et al., Radio-Frequency Losses of YBa2Cu3O7-δ Composite Superconductors, Supercond. Sci. Technol. 1, 24-26 (1988); High Tc Used in MRI, Supercond. Indus. 20 (Winter 1990); and Hall, et al., Use of High Temperature Superconductor in a Receiver Coil for Magnetic Resonance Imaging, Mag. Res. in Med. 20, 340-343 (1991).